1. Field of the Invention
The present invention relates to a touch panel sensor having electrodes, and more particularly to a touch panel sensor whose electrodes have a low resistance and has thinned conductive wires. The present invention also relates to a touch panel device including the touch panel sensor, and to a display device including the touch panel sensor or the touch panel device.
2. Description of Related Art
These days touch panel devices are widely used as input means. A touch panel device includes a touch panel sensor, a control circuit for detecting a touch position on the touch panel sensor, wiring and an FPC (flexible printed circuit) substrate. In many cases, a touch panel device is used, together with an image display mechanism, as an input means for a variety of devices (e.g. a ticket vending machine, an ATM device, a mobile phone, a gaming device, etc.) into which the image display mechanism such as a liquid crystal display or a plasma display is incorporated. In such a device, a touch panel sensor is disposed on the display surface of the image display mechanism; the touch panel device enables direct input to a display device. An area of the touch panel sensor which faces the display area of the image display mechanism is made transparent. The transparent area of the touch panel sensor constitutes an active area capable of detecting a contact position (approaching position).
Touch panel devices can be classified into various types based on the principle of detecting a contact position (approaching position) on a touch panel sensor. A capacitive-coupled touch panel device is currently attracting attention because of its optical brightness, good design, easy structure, excellent function, etc. In a capacitive-coupled touch panel device, a parasitic capacitance is newly produced when an external conductor (typically a finger) whose position is to be detected comes into contact with (approaches) a touch panel sensor via a dielectric body. The touch panel device utilizes a change in the capacitance to detect the position of the external conductor on the touch panel sensor.
The touch panel sensor usually includes a substrate and electrodes provided on the substrate. The electrodes include detection electrodes located in an active area and extraction electrodes located in a non-active area. In many touch panel sensors, detection electrodes are disposed at positions facing the display area of an image display mechanism, as disclosed e.g. in JP 2008-98169A, and therefore are formed of a transparent conductive material such as ITO. Such a transparent conductive material has a relatively high refractive index. This can produce a large difference in light transmittance and light reflectance between an area of a touch panel sensor where detection electrodes are disposed and an area where no detection electrode is disposed. In the case where there is a large difference in light transmittance and light reflectance between the two areas, the contour of the detection electrodes will be visible to the user of the touch panel sensor, which is undesirable from the viewpoint of design and, in addition, will significantly lower the image quality of the display device.
Another known touch panel sensor uses detection electrodes made of a metal material. The detection electrodes are composed of conductive wires with a narrow width. The active area of the touch panel sensor can therefore be made to have a sufficiently high light transmittance. Further, because of the high electrical conductivity of the metal material, the surface resistivity (unit: Ω/□) of the touch panel sensor can be made sufficiently low even when the metal wires each have a narrow width. Such a touch panel sensor is generally produced by first laminating a metal foil onto a transparent substrate using an adhesive, and then patterning the metal foil by etching using a photolithography technique to form electrodes.
As the image quality of display devices becomes higher and small portable terminals, called tablets, become widespread these days, there is a strong demand for the use of thinner electrodes.
However, in the conventional touch panel sensor which uses electrodes composed of metal wires, the electrodes are produced by etching a metal foil using a patterned resist as a mask. An industrially-manufactured metal foil has a thickness of not less than 10 μm. Metal wires, which can be stably produced by etching a metal foil of such a thickness, have a width of at least 10 μm. This is because, as shown in FIG. 21, lateral erosion (side etching) inevitably occurs during etching of the metal foil, whereby adjacent eroded portions connect with each other below the resist. Once eroded portions connect each other below the resist, the corresponding portion of the resist cannot be stably supported any more. Consequently, a metal wire formed below the resist portion lacks in linearity and varies in the height (thickness).
Further, as shown in FIG. 21, the resulting metal wires have a triangular cross-sectional shape projecting from the substrate. Such metal wires have a narrow width and a low height, and therefore do not have a sufficiently high conductivity. Accordingly, the touch panel sensor has a high surface resistivity and thus has a low sensing sensitivity for positional detection. In addition, when such a touch panel sensor is incorporated into a touch panel device or a display device such that the projecting metal wires face the viewer, the metal wires having a necessary surface resistivity are likely to be visible. Accordingly, a dark/light pattern due to the metal wires of the touch panel sensor as well as moire due to interference between the metal wires and a pixel array in an image display mechanism, metal wires of another touch panel sensor, etc., are likely to be visible.